The detailed study of cortical circuitry has been held back because anatomical techniques have not been available for selected single cell analysis with the electron microscope. Studies designed to analyze the connections of single cortical cells after physiological identification are now possible due to several new technical developments. One of these has been the discovery of a better intracellular marker, the enzyme horseradish peroxidase (HRP), that is visible in the light and electron microscope and suitable for intracellular marking of all dendritic processes of a single neuron. Another development is the refinement of bevelling techniques for glass microelectrodes that permit the penetration and injection of small (10-25 microns) neurons, such as occur in cerebral cortex. The proposed studies will take advantage of these developments to study selected inputs to cortical neurons after recording their physiological response properties. Specifically, neurons in somatc sensory-motor cortex of the opossum will be characterized by their response to direct electrical stimulation of afferent fiber systems, beginning with somatic afferents that relay through the ventrobasal nucleus. Following physiological characterization of a single neuron, the cell and its processes will be labeled by intracellular injection of HRP. This injected cell will be located in an area where the axon terminals of one selected afferent fiber system will have been tagged by another electron opaque marker which depends on active transport mechanisms. Reconstruction of serial electron micrographs through these injected cells will generate new information about the synaptic organization of single cortical neurons, as well as quantitative data about cortical connections.